The present invention relates to a method for controlling a measurement process of a coordinate measuring machine for measuring a measurement object, wherein the coordinate measuring machine comprises a controlling device and a feeler head having a feeler pin, and wherein a relative movement between the feeler pin and a surface of the measurement object is controlled by the controlling device.
The present invention furthermore relates to a coordinate measuring machine for measuring a measurement object, having a controlling device and a feeler head, which comprises a feeler pin, wherein a relative movement between the feeler pin and a surface of the measurement object is controlled by the controlling device.
Three-dimensional coordinate measuring machines are widely known in the prior art. In industrial applications, they are used to measure workpieces and, for example, thereby subject them to quality control. Such coordinate measuring machines are, however, also used in other fields of application, for example in the field of “reverse engineering”.
In order to measure the workpieces, various measuring systems are used. Conventionally, these are either optical measuring systems which allow contactless measuring of the workpieces, or tactile measuring systems in which the workpiece is sampled at particular points in order to detect the coordinates of the sampled points of the workpiece.
The present invention relates to coordinate measuring machines which comprise tactile measuring systems. Such a coordinate measuring machine has a feeler head, which is arranged movably in three dimensions relative to a workpiece in a measurement space. The feeler head comprises a sensor unit which supports a feeler pin in a mobile fashion and is capable of detecting a deflection of the feeler pin relative to the feeler head base. The feeler pin comprises a feeler tip on which a feeler object, generally a feeler ball, is provided. Owing to the known geometry of the feeler pin and the known position of the feeler head in the measurement space, spatial coordinates of the sampled measurement point on the workpiece can be determined with the aid of the deflection detected by the sensor unit.
In order to measure a workpiece, various types of measurement are known. The measuring may be carried out by sampling a multitude of points individually. Such a measurement method makes it possible, for example, to detect entirely unknown geometries, but requires increased time expenditure owing to the individual sampling of each measurement point.
Besides this, so-called scanning methods are also known, in which the feeler pin is displaced along the workpiece to be measured, in contact with the workpiece to be measured, and a multitude of measurement points along a path on the workpiece surface are thus detected. In this way, it is possible to detect a large number of measurement points very rapidly. For this, however, it is necessary to control the movement path of the feeler head so that the feeler pin, or the feeler ball, is constantly in contact with the surface of the workpiece. To this end, a setpoint geometry of the path is in general previously known. It may for example be known that the internal diameter of a circular bore is meant to be measured, so that the setpoint geometry is a circular path. Knowledge of this setpoint geometry facilitates the regulation of the path of the feeler head during the scanning process, so that adjustment of the path of the feeler head only has to be carried out with the aid of the actual deviation of the measurement points from the setpoint geometry.
Furthermore, so-called actively measuring and so-called passively measuring feeler heads are known in the prior art. In the case of passively measuring feeler heads, the feeler pin is essentially supported by means of mechanical spring elements. An deflection of the feeler pin is therefore always proportional to a spring force of the bearing and therefore to a feeler force, with which the feeler pin presses against the workpiece surface. During a scanning process along a workpiece surface, it is advantageous for the feeler force only to lie in a particular range, in order to avoid excessive bending of the feeler pin or damage to the workpiece. Since with passively measuring feeler heads the feeler force is proportional to the deflection of the feeler pin, in this case the feeler head needs to be moved in order to change the deflection of the feeler pin and therefore the feeler force. Movement of the feeler head entails accelerations and therefore inertial forces, which can make it more difficult to control such a passively measuring feeler head.
In the case of actively measuring feeler heads, the feeler force can be set independently of the deflection of the feeler pin, with the aid of measuring force transducers (for example electric motor-driven plunger coils or piezo elements). Even in the event of large deflections of the feeler pin, this makes it possible to sample the workpiece with a small feeler force and/or a feeler force which is constant over the course of a scanning process, without this entailing excessive movement of the feeler head, and sometimes even with no movement of the feeler head.
The present invention is primarily intended to be used with actively measuring feeler heads. In principle, however, it may also be employed for passively measuring feeler heads.
Scanning methods can quite readily be used when smooth surfaces are to be sampled. If the surfaces to be sampled comprise elevations or depressions, however, for instance milled grooves, problems may arise when the feeler pin with its feeler ball catches on an elevation or in a depression. Depressions in workpiece surfaces may in particular constitute a problem here. Under certain circumstances, however, it is not actually necessary to measure the depression in the workpiece surface accurately, since what is important is only the profile of the surface in which this depression is formed, and for example whether the surface is actually circular, or flat.
The basic terms of coordinate measuring technology, and in particular methods for evaluating the measurement results, are known to the person skilled in the art. Information about known methods of coordinate measuring technology, and in particular about interpolation methods, may be found in the textbook Weckenmann, Gawande, “Koordinatenmeβtechnik”, Carl Hanser Verlag, Munich, 1999, ISBN 3-446-17991-7, to which reference is explicitly made here in this regard.
One problem which often arises is that as soon as the feeler pin moves over a depression in a surface, a feeler ball of the feeler pin is adjusted by a controlling device so that the feeler pin enters the depression. The feeler ball so to speak “falls” into the depression. Subsequently, when the groove ends, the feeler pin then comes in contact with the opposite edge of the groove. Since the scanning process is carried out with a particular rate of advance, for example five millimeters per second, it is possible that the feeler pin can no longer be “retracted” from the groove rapidly enough. As a result, either the feeler pin may be damaged or the scanning process will need to be interrupted so that the controlling device of the coordinate measuring machine can first release the feeler pin from the depression or groove.
Possible solutions for avoiding such “entry” of the feeler pin into depressions of a surface to be measured have therefore been sought.
For instance, U.S. Pat. No. 5,895,444 (A), which constitutes a development of U.S. Pat. No. 5,737,244 (A), proposes that, with approximate knowledge of the position of a workpiece depression, a negative virtual measuring force be set. “Negative” in this case means that the measuring force points in the normal direction away from the workpiece surface to be measured. The controlling device will then assume that the feeler force is too great and will lift the feeler ball from the workpiece surface. In this way, it is possible to “jump” over the depression. After the depression has been crossed over, the virtually provided force is cancelled and the controlling device continues to sample the workpiece surface. This, however, requires that the approximate position of the depressions be known.
It is also proposed in WO 2008/074989 A1 to lift the feeler ball from the workpiece surface when approaching a depression, in order to avoid entry and catching of the feeler ball, or the feeler pin, in a depression. Here again, the approximate position of a groove or depression must be known.
It is an object of the present invention to provide a way of scanning a workpiece surface comprising depressions, which makes scanning possible even if there is in fact no workpiece at a measurement position, for example because of a depression. In this context, the intention is for all known measurement, scanning and interpolation methods to remain usable without modifications, and to prevent a feeler pin from catching in the depression even if there is no knowledge about depressions which are present. Furthermore, it must still be possible to identify whether a detected measurement value derives from the workpiece surface actually being scanned.